Dissolved Oxygen and Henry's LawOxygen dissolved in water can come from the atmosphere or as a byproduct of the photosynthesis of aquatic plants.
For water in equilibrium with the atmosphere, the concentration is governed by Henry's Law, and KH for O2 is 769.23. We can calculate the concentration of O2(aq) to be 0.00027 M. Typically, BOD is measured in mg/L or in percent saturation rather than mol/L. Percent saturation is the amount of oxygen in a liter of water relative to the total amount of oxygen that the water can hold at that temperature.
Water below the air/water interface is not necessarily in equilibrium with the air and can have even less oxygen than this small value. In fact, it is typical for the oxygen concentration in lakes and in the ocean to decrease with depth in the water.
Running water in shallow streams mixes better with air and tends to have a higher oxygen content than still water. There is also a strong temperature dependence on maximum oxygen concentration (see the table below). Atmospheric pressure is lower at higher altitudes so water at higher elevations holds less dissolved oxygen than water at sea level.
The dissolved oxygen levels are higher in the summer and during daylight hours because this is when photosynthetic organisms produce oxygen. Thermal discharges, such as water used to cool machinery in a manufacturing plant or a power plant, raise the temperature of water and lower its oxygen content. Aquatic animals are most vulnerable to lowered dissolved oxygen levels in the early morning on hot summer days when stream flows are low, water temperatures are high, and aquatic plants have not begun producing oxygen.
Biological Oxygen DemandThis is a measure of the amount of molecular oxygen in milligrams required to convert organic molecules contained in 1.0 liter of a water sample to CO2.
Microorganisms such as bacteria are responsible for decomposing organic waste. When organic matter such as dead plants, leaves, grass clippings, manure, sewage, or even food waste is present in a water supply, the bacteria will begin the process of breaking down this waste. When this happens, much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they need to live. BOD level is a common metric for water pollution.
The BOD level is determined by comparing the dissolved oxygen levels of a water sample before and after 5 days of incubation in the dark (see the next section for this). The difference between the two DO levels represents the amount of oxygen required for the decomposition of any organic material in the sample and is a good approximation of the BOD level.
BOD values are usually reported in either ppm values or in mg/L values. Unpolluted rivers typically have a BOD below 1 mg/L. Moderately polluted rivers vary between 2 to 8 mg/L. Untreated sewage averages between 200 and 600 mg/L while efficiently treated municipal sewage would be 20 mg/L or less.
Chemical Oxygen DemandThis is another measure of the oxidizable organic compounds in water. In this method, organic materials are oxidized to CO2 with dichromate in acidic solution. Dichromate doesn't oxidize ammonia and this can give a reading that underestimates the biological oxygen demand. Ammonia decomposition to nitrate uses molecular oxygen. Dichromate can oxidize other inorganic components of water, and this produces a BOD estimate that is too high.
In a COD measurement, a known, excess amount of potassium dichromate and sulfuric acid are added to the sample. The oxidizable material reacts with this reagent and Cr(IV) is converted to Cr(III).
Then the excess potassium dichromate is titrated with ferrous ammonium sulfate until all of the excess oxidizing agent has been reduced to Cr(III). Ferroin indicator is added and the color changes from blue-green to reddish-brown at the end point.